.\" Copyright 2002 Walter Harms <walter.harms@informatik.uni-oldenburg.de>
.\" and Andries Brouwer <aeb@cwi.nl>.
.\"
.\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
.\" Distributed under GPL
.\" %%%LICENSE_END
.\"
.TH IOCTL_TTY 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
ioctl_tty \- ioctls for terminals and serial lines
.SH SYNOPSIS
.nf
.B "#include <termios.h>"
.PP
.BI "int ioctl(int " fd ", int " cmd ", ...);"
.fi
.SH DESCRIPTION
The
.BR ioctl (2)
call for terminals and serial ports accepts many possible command arguments.
Most require a third argument, of varying type, here called
.I argp
or
.IR arg .
.PP
Use of
.I ioctl
makes for nonportable programs.
Use the POSIX interface described in
.BR termios (3)
whenever possible.
.SS Get and set terminal attributes
.TP
.B TCGETS
Argument:
.BI "struct termios *" argp
.IP
Equivalent to
.IR "tcgetattr(fd, argp)" .
.IP
Get the current serial port settings.
.TP
.B TCSETS
Argument:
.BI "const struct termios *" argp
.IP
Equivalent to
.IR "tcsetattr(fd, TCSANOW, argp)" .
.IP
Set the current serial port settings.
.TP
.B TCSETSW
Argument:
.BI "const struct termios *" argp
.IP
Equivalent to
.IR "tcsetattr(fd, TCSADRAIN, argp)" .
.IP
Allow the output buffer to drain, and
set the current serial port settings.
.TP
.B TCSETSF
Argument:
.BI "const struct termios *" argp
.IP
Equivalent to
.IR "tcsetattr(fd, TCSAFLUSH, argp)" .
.IP
Allow the output buffer to drain, discard pending input, and
set the current serial port settings.
.PP
The following four ioctls are just like
.BR TCGETS ,
.BR TCSETS ,
.BR TCSETSW ,
.BR TCSETSF ,
except that they take a
.I "struct termio\ *"
instead of a
.IR "struct termios\ *" .
.IP
.BI "TCGETA	struct termio *" argp
.IP
.BI "TCSETA	const struct termio *" argp
.IP
.BI "TCSETAW	const struct termio *" argp
.IP
.BI "TCSETAF	const struct termio *" argp
.SS Locking the termios structure
The
.I termios
structure of a terminal can be locked.
The lock is itself a
.I termios
structure, with nonzero bits or fields indicating a
locked value.
.TP
.B TIOCGLCKTRMIOS
Argument:
.BI "struct termios *" argp
.IP
Gets the locking status of the
.I termios
structure of the terminal.
.TP
.B TIOCSLCKTRMIOS
Argument:
.BI "const struct termios *" argp
.IP
Sets the locking status of the
.I termios
structure of the terminal.
Only a process with the
.BR CAP_SYS_ADMIN
capability can do this.
.SS Get and set window size
Window sizes are kept in the kernel, but not used by the kernel
(except in the case of virtual consoles, where the kernel will
update the window size when the size of the virtual console changes,
for example, by loading a new font).
.PP
The following constants and structure are defined in
.IR <sys/ioctl.h> .
.TP
.B TIOCGWINSZ
Argument:
.BI "struct winsize *" argp
.IP
Get window size.
.TP
.B TIOCSWINSZ
Argument:
.BI "const struct winsize *" argp
.IP
Set window size.
.PP
The struct used by these ioctls is defined as
.PP
.in +4n
.EX
struct winsize {
    unsigned short ws_row;
    unsigned short ws_col;
    unsigned short ws_xpixel;   /* unused */
    unsigned short ws_ypixel;   /* unused */
};
.EE
.in
.PP
When the window size changes, a
.B SIGWINCH
signal is sent to the
foreground process group.
.SS Sending a break
.TP
.B TCSBRK
Argument:
.BI "int " arg
.IP
Equivalent to
.IR "tcsendbreak(fd, arg)" .
.IP
If the terminal is using asynchronous serial data transmission, and
.I arg
is zero, then send a break (a stream of zero bits) for between
0.25 and 0.5 seconds.
If the terminal is not using asynchronous
serial data transmission, then either a break is sent, or the function
returns without doing anything.
When
.I arg
is nonzero, nobody knows what will happen.
.IP
(SVr4, UnixWare, Solaris, Linux treat
.I "tcsendbreak(fd,arg)"
with nonzero
.I arg
like
.IR "tcdrain(fd)" .
SunOS treats
.I arg
as a multiplier, and sends a stream of bits
.I arg
times as long as done for zero
.IR arg .
DG/UX and AIX treat
.I arg
(when nonzero) as a time interval measured in milliseconds.
HP-UX ignores
.IR arg .)
.TP
.B TCSBRKP
Argument:
.BI "int " arg
.IP
So-called "POSIX version" of
.BR TCSBRK .
It treats nonzero
.I arg
as a time interval measured in deciseconds, and does nothing
when the driver does not support breaks.
.TP
.B TIOCSBRK
Argument:
.BI "void"
.IP
Turn break on, that is, start sending zero bits.
.TP
.B TIOCCBRK
Argument:
.BI "void"
.IP
Turn break off, that is, stop sending zero bits.
.SS Software flow control
.TP
.B TCXONC
Argument:
.BI "int " arg
.IP
Equivalent to
.IR "tcflow(fd, arg)" .
.IP
See
.BR tcflow (3)
for the argument values
.BR TCOOFF ,
.BR TCOON ,
.BR TCIOFF ,
.BR TCION .
.SS Buffer count and flushing
.TP
.BI FIONREAD
Argument:
.BI "int *" argp
.IP
Get the number of bytes in the input buffer.
.TP
.B TIOCINQ
Argument:
.BI "int *" argp
.IP
Same as
.BR FIONREAD .
.TP
.B TIOCOUTQ
Argument:
.BI "int *" argp
.IP
Get the number of bytes in the output buffer.
.TP
.B TCFLSH
Argument:
.BI "int " arg
.IP
Equivalent to
.IR "tcflush(fd, arg)" .
.IP
See
.BR tcflush (3)
for the argument values
.BR TCIFLUSH ,
.BR TCOFLUSH ,
.BR TCIOFLUSH .
.SS Faking input
.TP
.B TIOCSTI
Argument:
.BI "const char *" argp
.IP
Insert the given byte in the input queue.
.SS Redirecting console output
.TP
.B TIOCCONS
Argument:
.BI "void"
.IP
Redirect output that would have gone to
.I /dev/console
or
.I /dev/tty0
to the given terminal.
If that was a pseudoterminal master, send it to the slave.
In Linux before version 2.6.10,
anybody can do this as long as the output was not redirected yet;
since version 2.6.10, only a process with the
.BR CAP_SYS_ADMIN
capability may do this.
If output was redirected already, then
.B EBUSY
is returned,
but redirection can be stopped by using this ioctl with
.I fd
pointing at
.I /dev/console
or
.IR /dev/tty0 .
.SS Controlling terminal
.TP
.B TIOCSCTTY
Argument:
.BI "int " arg
.IP
Make the given terminal the controlling terminal of the calling process.
The calling process must be a session leader and not have a
controlling terminal already.
For this case,
.I arg
should be specified as zero.
.IP
If this terminal is already the controlling terminal
of a different session group, then the ioctl fails with
.BR EPERM ,
unless the caller has the
.BR CAP_SYS_ADMIN
capability and
.I arg
equals 1, in which case the terminal is stolen, and all processes that had
it as controlling terminal lose it.
.TP
.B TIOCNOTTY
Argument:
.BI "void"
.IP
If the given terminal was the controlling terminal of the calling process,
give up this controlling terminal.
If the process was session leader,
then send
.B SIGHUP
and
.B SIGCONT
to the foreground process group
and all processes in the current session lose their controlling terminal.
.SS Process group and session ID
.TP
.B TIOCGPGRP
Argument:
.BI "pid_t *" argp
.IP
When successful, equivalent to
.IR "*argp = tcgetpgrp(fd)" .
.IP
Get the process group ID of the foreground process group on this terminal.
.TP
.B TIOCSPGRP
Argument:
.BI "const pid_t *" argp
.IP
Equivalent to
.IR "tcsetpgrp(fd, *argp)" .
.IP
Set the foreground process group ID of this terminal.
.TP
.B TIOCGSID
Argument:
.BI "pid_t *" argp
.IP
Get the session ID of the given terminal.
This fails with the error
.B ENOTTY
if the terminal is not a master pseudoterminal
and not our controlling terminal.
Strange.
.SS Exclusive mode
.TP
.B TIOCEXCL
Argument:
.BI "void"
.IP
Put the terminal into exclusive mode.
No further
.BR open (2)
operations on the terminal are permitted.
(They fail with
.BR EBUSY ,
except for a process with the
.BR CAP_SYS_ADMIN
capability.)
.TP
.B TIOCGEXCL
Argument:
.BI "int *" argp
.IP
(since Linux 3.8)
If the terminal is currently in exclusive mode,
place a nonzero value in the location pointed to by
.IR argp ;
otherwise, place zero in
.IR *argp .
.TP
.B TIOCNXCL
Argument:
.BI "void"
.IP
Disable exclusive mode.
.SS Line discipline
.TP
.B TIOCGETD
Argument:
.BI "int *" argp
.IP
Get the line discipline of the terminal.
.TP
.B TIOCSETD
Argument:
.BI "const int *" argp
.IP
Set the line discipline of the terminal.
.SS Pseudoterminal ioctls
.TP
.B TIOCPKT
Argument:
.BI "const int *" argp
.IP
Enable (when
.RI * argp
is nonzero) or disable packet mode.
Can be applied to the master side of a pseudoterminal only (and will return
.B ENOTTY
otherwise).
In packet mode, each subsequent
.BR read (2)
will return a packet that either contains a single nonzero control byte,
or has a single byte containing zero (\(aq\e0\(aq) followed by data
written on the slave side of the pseudoterminal.
If the first byte is not
.B TIOCPKT_DATA
(0), it is an OR of one
or more of the following bits:
.IP
.ad l
.TS
lb l.
TIOCPKT_FLUSHREAD	T{
The read queue for the terminal is flushed.
T}
TIOCPKT_FLUSHWRITE	T{
The write queue for the terminal is flushed.
T}
TIOCPKT_STOP	T{
Output to the terminal is stopped.
T}
TIOCPKT_START	T{
Output to the terminal is restarted.
T}
TIOCPKT_DOSTOP	T{
The start and stop characters are \fB\(haS\fP/\fB\(haQ\fP.
T}
TIOCPKT_NOSTOP	T{
The start and stop characters are not \fB\(haS\fP/\fB\(haQ\fP.
T}
.TE
.ad
.IP
While packet mode is in use, the presence
of control status information to be read
from the master side may be detected by a
.BR select (2)
for exceptional conditions or a
.BR poll (2)
for the
.B POLLPRI
event.
.IP
This mode is used by
.BR rlogin (1)
and
.BR rlogind (8)
to implement a remote-echoed,
locally \fB\(haS\fP/\fB\(haQ\fP flow-controlled remote login.
.TP
.B TIOCGPKT
Argument:
.BI "const int *" argp
.IP
(since Linux 3.8)
Return the current packet mode setting in the integer pointed to by
.IR argp .
.TP
.B TIOCSPTLCK
Argument:
.BI "int *" argp
.IP
Set (if
.IR *argp
is nonzero) or remove (if
.IR *argp
is zero) the lock on the pseudoterminal slave device.
(See also
.BR unlockpt (3).)
.TP
.B TIOCGPTLCK
Argument:
.BI "int *" argp
.IP
(since Linux 3.8)
Place the current lock state of the pseudoterminal slave device
in the location pointed to by
.IR argp .
.TP
.B TIOCGPTPEER
Argument:
.BI "int " flags
.IP
.\" commit 54ebbfb1603415d9953c150535850d30609ef077
(since Linux 4.13)
Given a file descriptor in
.I fd
that refers to a pseudoterminal master,
open (with the given
.BR open (2)-style
.IR flags )
and return a new file descriptor that refers to the peer
pseudoterminal slave device.
This operation can be performed
regardless of whether the pathname of the slave device
is accessible through the calling process's mount namespace.
.IP
Security-conscious programs interacting with namespaces may wish to use this
operation rather than
.BR open (2)
with the pathname returned by
.BR ptsname (3),
and similar library functions that have insecure APIs.
(For example, confusion can occur in some cases using
.BR ptsname (3)
with a pathname where a devpts filesystem
has been mounted in a different mount namespace.)
.PP
The BSD ioctls
.BR TIOCSTOP ,
.BR TIOCSTART ,
.BR TIOCUCNTL ,
.B TIOCREMOTE
have not been implemented under Linux.
.SS Modem control
.TP
.B TIOCMGET
Argument:
.BI "int *" argp
.IP
Get the status of modem bits.
.TP
.B TIOCMSET
Argument:
.BI "const int *" argp
.IP
Set the status of modem bits.
.TP
.B TIOCMBIC
Argument:
.BI "const int *" argp
.IP
Clear the indicated modem bits.
.TP
.B TIOCMBIS
Argument:
.BI "const int *" argp
.IP
Set the indicated modem bits.
.PP
The following bits are used by the above ioctls:
.PP
.TS
lb l.
TIOCM_LE	DSR (data set ready/line enable)
TIOCM_DTR	DTR (data terminal ready)
TIOCM_RTS	RTS (request to send)
TIOCM_ST	Secondary TXD (transmit)
TIOCM_SR	Secondary RXD (receive)
TIOCM_CTS	CTS (clear to send)
TIOCM_CAR	DCD (data carrier detect)
TIOCM_CD	see TIOCM_CAR
TIOCM_RNG	RNG (ring)
TIOCM_RI	see TIOCM_RNG
TIOCM_DSR	DSR (data set ready)
.TE
.TP
.B TIOCMIWAIT
Argument:
.BI "int " arg
.IP
Wait for any of the 4 modem bits (DCD, RI, DSR, CTS) to change.
The bits of interest are specified as a bit mask in
.IR arg ,
by ORing together any of the bit values,
.BR TIOCM_RNG ,
.BR TIOCM_DSR ,
.BR TIOCM_CD ,
and
.BR TIOCM_CTS .
The caller should use
.B TIOCGICOUNT
to see which bit has changed.
.TP
.B TIOCGICOUNT
Argument:
.BI "struct serial_icounter_struct *" argp
.IP
Get counts of input serial line interrupts (DCD, RI, DSR, CTS).
The counts are written to the
.I serial_icounter_struct
structure pointed to by
.IR argp .
.IP
Note: both 1->0 and 0->1 transitions are counted, except for
RI, where only 0->1 transitions are counted.
.SS Marking a line as local
.TP
.B TIOCGSOFTCAR
Argument:
.BI "int *" argp
.IP
("Get software carrier flag")
Get the status of the CLOCAL flag in the c_cflag field of the
.I termios
structure.
.TP
.B TIOCSSOFTCAR
Argument:
.BI "const int *" argp
.IP
("Set software carrier flag")
Set the CLOCAL flag in the
.I termios
structure when
.RI * argp
is nonzero, and clear it otherwise.
.PP
If the
.B CLOCAL
flag for a line is off, the hardware carrier detect (DCD)
signal is significant, and an
.BR open (2)
of the corresponding terminal will block until DCD is asserted,
unless the
.B O_NONBLOCK
flag is given.
If
.B CLOCAL
is set, the line behaves as if DCD is always asserted.
The software carrier flag is usually turned on for local devices,
and is off for lines with modems.
.SS Linux-specific
For the
.B TIOCLINUX
ioctl, see
.BR ioctl_console (2).
.SS Kernel debugging
.B "#include <linux/tty.h>"
.TP
.B TIOCTTYGSTRUCT
Argument:
.BI "struct tty_struct *" argp
.IP
Get the
.I tty_struct
corresponding to
.IR fd .
This command was removed in Linux 2.5.67.
.\"     commit b3506a09d15dc5aee6d4bb88d759b157016e1864
.\"     Author: Andries E. Brouwer <andries.brouwer@cwi.nl>
.\"     Date:   Tue Apr 1 04:42:46 2003 -0800
.\"
.\"     [PATCH] kill TIOCTTYGSTRUCT
.\"
.\"     Only used for (dubious) debugging purposes, and exposes
.\"     internal kernel state.
.\"
.\" .SS Serial info
.\" .BR "#include <linux/serial.h>"
.\" .PP
.\" .TP
.\" .BI "TIOCGSERIAL	struct serial_struct *" argp
.\" Get serial info.
.\" .TP
.\" .BI "TIOCSSERIAL	const struct serial_struct *" argp
.\" Set serial info.
.SH RETURN VALUE
The
.BR ioctl (2)
system call returns 0 on success.
On error, it returns \-1 and sets
.I errno
to indicate the error.
.SH ERRORS
.TP
.B EINVAL
Invalid command parameter.
.TP
.B ENOIOCTLCMD
Unknown command.
.TP
.B ENOTTY
Inappropriate
.IR fd .
.TP
.B EPERM
Insufficient permission.
.SH EXAMPLES
Check the condition of DTR on the serial port.
.PP
.EX
#include <termios.h>
#include <fcntl.h>
#include <sys/ioctl.h>

int
main(void)
{
    int fd, serial;

    fd = open("/dev/ttyS0", O_RDONLY);
    ioctl(fd, TIOCMGET, &serial);
    if (serial & TIOCM_DTR)
        puts("TIOCM_DTR is set");
    else
        puts("TIOCM_DTR is not set");
    close(fd);
}
.EE
.SH SEE ALSO
.BR ldattach (1),
.BR ioctl (2),
.BR ioctl_console (2),
.BR termios (3),
.BR pty (7)
.\"
.\" FIONBIO			const int *
.\" FIONCLEX			void
.\" FIOCLEX			void
.\" FIOASYNC			const int *
.\" from serial.c:
.\" TIOCSERCONFIG		void
.\" TIOCSERGWILD		int *
.\" TIOCSERSWILD		const int *
.\" TIOCSERGSTRUCT		struct async_struct *
.\" TIOCSERGETLSR		int *
.\" TIOCSERGETMULTI		struct serial_multiport_struct *
.\" TIOCSERSETMULTI		const struct serial_multiport_struct *
.\" TIOCGSERIAL, TIOCSSERIAL (see above)
